Historically, chemical mechanical planarization, also known as "cmp," has been developed for many uses in the semiconductor industry; like removing a thin top layer of material from a semiconductor wafer.
A representative cmp machine, as illustrated in FIG. 1, comprises: a wafer carrier 10 holding a semiconductor wafer 12 which is pressed against a course polishing pad 16 mounted on a platen or holder 18 which rotates about its axis. Not shown in this figure, is the chemical slurry which is applied to the pad 16 for using chemical action to further aid in removing material from the surface of the semiconductor wafer 12.
In operation, a cmp machine works like a sanding machine and acid bath combined. To mechanically remove wafer material, wafer 12 is positionally fixed and held against abrasive polishing pad 16 while platen or holder 18 rotates. The chemical slurry is used to chemically remove the wafer material.
FIG. 2 illustrates a representative cross section of a wafer 12 comprising: a tungsten material 20 having a top surface layer 23, a few silicon-type material sections 22, and a wafer base 24.
FIG. 3 illustrates the desired resulting wafer design desired after removing the appropriate amount of top layer material 23; thus, leaving a smooth and planar top surface.
Problems
A major problem with current cmp processes is that FIG. 3 is a goal that is achievable only by a long process time; usually requiring seven or more minutes. As illustrated in FIG. 4, it is shown that material 20 has not achieved a level surface that is even with the silicon-type material sections 22. this situation occurs because the chemical removal process usually continues to remove the tungsten even after the mechanical material removal process has ceased. It has been difficult to control the chemical removal process. The option of discarding the chemical process is not possible, it being the fastest removal process. Therefore, complex timing schemes have been developed for removing the wafers before the chemical process removes too much tungsten.
It is noted that the above described problem, as well as other problems, are solved through the subject invention and will become more apparent, to one skilled in the art, from the detailed description of the subject invention.